Heartworm Disease in Cats

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By: Kari Lyon, VMD

Despite what most people think, heartworm disease is not only a disease of our canine friends, it affects
our feline friends as well, albeit much less commonly. Heartworm disease in cats has increased in
incidence and awareness over the past few years. Heartworm disease in cats in a problem anywhere it is
a problem in dogs.
Cats are infected with heartworms the same way dogs are, by the bite of an infected mosquito. The
parasite responsible for the disease is the same as well, Dirofilaria Immitis. However, there are more
differences than similarities between canine and feline heartworm disease. Despite the number of larvae
injected into the cats blood stream by the mosquito, very few live to be adult worms in cats, whereas
approximately 95% grow into adults in dogs. The adult worms generally only live about 2-3 years in cats
and can live to be 5 or 6 years old in a dog. Both indoor and outdoor cats are at risk for heartworm
A cat’s body reacts differently to the presence of immature and adult heartworms than a dogs body does.
In dogs, the problems associated with heartworm infection are usually due to the presence of such large
worms within the dog’s vessels causing obstruction of blood flow. The cat’s disease is due to a severe
inflammatory response to the presence of the immature worms. For this reason it is very difficult to
diagnose heartworm disease in cats because their body gets rid of the immature worms very quickly.
Symptoms of heartworm disease in cats are more associated with lung disease rather than heart disease
as it is in dogs. Such symptoms include lethargy, vomiting collapse, seizures, respiratory problems, as
well as sudden death.
As previously mentioned, diagnosing heartworm disease can be very difficult. Because the disease is not
dependent on adult worms, the test used in dogs (antigen test) is not very helpful. The second test
commonly used in dogs looks for the presence of immature worms in the blood. This test too cannot be
used in cats because cats usually rid the body of immature worms quickly, they usually do not have
enough adults to produce offspring or there may be only a few adults of a single sex. In cats, testing for
antibodies against D. Immitis can be helpful but is not 100% diagnostic. A positive test may indicate an
infection with only mature and/or immature worms, or it could indicate a past infection. For this reason the
American Heartworm Association recommends combining an antigen and antibody test when screening
apparently healthy cats. In cats where signs of heart and /or lung disease are present or heartworm
disease is suspected, both these tests should be performed along with chest radiographs and
Treatment of heartworm disease in cats is usually aimed at decreasing the body’s inflammatory response
to the present worms using steroids. In cat’s that are not showing clinical signs of disease, the American
Heartworm Society recommends waiting out the worm’s 2 to 3 year life span whiling monitoring chest
radiographs every 6 months for signs of worsening disease. Cats that become acutely ill due to
heartworm disease need to be stabilized prior to treatment. At this point in time there are no treatments
licensed for cats that are affective at killing adult heartworms, and there are no studies to confirm that
killing the adult heartworms increases survival rates.
Heartworm disease in cats is 100% preventable as it is dogs. There are monthly preventative products
available to cats for the prevention if heartworm disease. Please ask your veterinarian which product
would be right for you and your cat.

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